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Homework Help: Gauss's Law and charged paint

  1. Sep 22, 2013 #1
    1. The problem statement, all variables and given/known data
    A charged paint is spread in a very thin uniform layer over the surface of a plastic sphere of diameter 10.5 cm, giving it a charge of -22.3 µC.

    (a) Find the magnitude of the electric field just inside the paint layer.
    (b) Find the magnitude of the electric field just outside the paint layer.
    (c) Find the magnitude of the electric field 5.00 cm outside the surface of the paint layer.

    2. Relevant equations
    $$E=\frac{q}{4\pi\epsilon_0 r^2}$$

    3. The attempt at a solution
    I got (a) correct (E=0), but I'm getting wrong answers for (b) and (c)

    For (b), the radius is 0.0525m, so:
    $$E = \frac{-22.3\times 10^{-6}}{4\pi (0.1025)^2(8.854\times 10^{-12})} = -7.272\times 10^7$$

    For (c), r=0.0525+0.05=0.1025, so:
    $$E = \frac{-22.3\times 10^{-6}}{4\pi (0.1025)^2(8.854\times 10^{-12})} = -7.272\times 10^7$$

    EDIT: for some reason my latex code isn't displaying properly, so here are pics of my equations:
    EDIT 2: fixed it
    Last edited: Sep 22, 2013
  2. jcsd
  3. Sep 22, 2013 #2

    Simon Bridge

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    Science Advisor
    Homework Helper

    2. Relevant equations
    $$E=\frac{q}{4\pi\epsilon_0 r^2}$$

    For (b), the radius is 0.0525m, so:$$E = \frac{-22.3\times 10^{-6}}{4\pi (0.0525)^2(8.854\times 10^{-12})} = -7.272\times 10^7$$
    Code (Text):
    > (-22.3e-6)/(4*pi*(0.0525^2)*(8.854e-12))
    ans =  -7.2717e+07
    ... hmmm, if this is being computer mediated, then you should check rounding and the exact form of the input, units, stuff like that. Perhaps they'll accept ##\small -7.27\times 10^7##V/m

    That's because you didn't use only latex markup inside the latex tags.
    Use the "quote" button (below, right) to see what I did.
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